To create a global variable in TypeScript, we have multiple approaches. In this article, we are going to learn how to create a global variable in TypeScript.

Below are the approaches used to create a global variable in TypeScript:

Using var, let or const at the Top Level

Declare a variable with var, let or const at the top level of your script.

Example: In this example, we are using var, let, and const to create a global variable in TypeScript.

var globalVar: string = "I am a global variable using var";
let globalLet: string = "I am a global variable using let";
const globalConst: number = 42;

function useGlobalVarLetAndConst() {
    console.log(globalLet, globalConst, globalVar);
}

useGlobalVarLetAndConst();  // Accessible here

Output:

I am a global variable using let 
42
I am a global variable using var

Using window Object in Browser Environments

Assign a property to the window object for global accessibility in browsers.

Example: In this example, we are using window Object in Browser Environments to create a global variable in TypeScript.

// Using the window object in browser environments
(window as any).browserGlobal =
    "I am a global variable accessible in browsers";

function useBrowserGlobal() {
    console.log((window as any).browserGlobal);
}

useBrowserGlobal();  // Accessible here (in browser environments)

Output:

I am a global variable accessible in browsers

Using a Namespace or Module

Place the variable inside a namespace or module for organized global scope.

Example: In this example, we are using a Namespace or Module to create a global variable in TypeScript.

// Using a namespace or module
namespace GlobalNamespace {
    export let globalVariable: string = "I am a global variable";
}

function useGlobalNamespace() {
    console.log(GlobalNamespace.globalVariable);
}

useGlobalNamespace();  // Accessible here

// Using a namespace or module

namespace GlobalNamespace {

    export let globalVariable: string = "I am a global variable";

}

​

function useGlobalNamespace() {

    console.log(GlobalNamespace.globalVariable);

}

​

useGlobalNamespace();  // Accessible here

Output:

I am a global variable

Using declare Keyword

Use the declare keyword for external global variables without a specific implementation.

Example: In this example, we are using declare Keyword to create a global variable in TypeScript.

// Using declare keyword
declare var declaredGlobal: boolean;

function useDeclaredGlobal() {
    console.log(declaredGlobal);
}

useDeclaredGlobal();  // Accessible here

Output:

The output will depend on the actual value assigned to externalLibraryVar at runtime. For example, if it is defined externally before calling useExternalVariable(), the output might look like this:

[declaredGlobal value]

If declaredGlobal is not defined, you might encounter a runtime error similar to:

Uncaught ReferenceError: declaredGlobal is not defined